Conventional water tube package boilers, such as that described in U.S. Pat. No. 4,528,945 (Virr et al.), which issued on Jul. 16, 1985, and which is incorporated herein by reference, are typically used to burn coal and other carbonaceous material in an effort to heat water or another liquid to generate stream or another vapor. Water tube package boilers have two main compartments, a combustion chamber comprising a bed of particles which are fluidized when the boiler is in use and an adjacent heat exchange chamber comprising numerous vertically disposed heat exchange tubes. It is desirable that all of the carbonaceous material be combusted within the fluid bed and freeboard regions of the combustion chamber prior to exiting therefrom for processing in the heat exchange chamber. According to U.S. Pat. No. 4,528,945 the fluid bed region of the combustion chamber has a sloped configuration such that there is a shallower part which can be fluidized independently of the deeper part of the fluid bed so that the rate of heat transfer from the bed to the tubes can be varied according to demand.
Unfortunately, conventional package boilers such as that disclosed in U.S. Pat. No. 4,528,945 have certain disadvantages which can be summarized as follows: (1) they are an integral boiler design in which the in-bed tubes are an integral part of the side walls; (2) they can only be fed with fuel from either the roof or end walls of the combustion chamber; (3) they have an insufficient freeboard region which causes unburned carbonaceous particles to be carried out of the combustion chamber together with the discharged gases which seriously impairs combustion efficiency; (4) such boilers do not have any means to recycle entrained particles which pass into the heat exchange chamber; (5) the circulation baffles disposed within the fluid bed for the purpose of directing the flow of the fluid bed from the center to the side wall are subjected to extensive deterioration and erosion regardless of whether they are formed from metal or refractory; and (6) such boilers typically require that the distribution nozzles or tuyeres disposed nearest the side walls on the bottom of the fluid bed region be taller than those disposed towards the center of the combustion chamber.
U.S. Pat. No. 5,005,528 (Virr), which issued on Apr. 9, 1991, attempted to overcome the deficiencies of the conventional package boiler design disclosed in U.S. Pat. No. 4,528,945, but again it include undesirable integral side wall in-bed tubes and uncooled fluid bed circulation baffles. It also requires that the main combustion chamber operate at 8 to 17 feet/second which cause substantial recycle, thus requiring the use of expensive recycle cyclones and additional freeboard height in order to get optimum combustion efficiency.
The addition of a cyclone and additional freeboard is extremely impractical for previously installed water tube package boilers due to the high cost associated with such a retrofit and the lack of available physical space needed for the installation of such large pieces of equipment.
The present invention is uniquely designed to be incorporated into or bolted onto the bottom portion of a combustion chamber in the space previously allocated for a stoker or gas/oil boiler. This conversion fluid bed chamber assembly effectively extends the freeboard region of the combustion chamber without requiring additional height extension to the top of the combustion chamber, provides for cooling of the bed circulation baffles, the in-bed heat exchange tubes are no longer necessarily integral to the side wall tubes, solid particles which are entrained with the discharged gases that flow into the adjacent heat exchange chamber are recycled to the combustion chamber without the need for an expensive cyclone, and the carbonaceous material can be fed into the combustion chamber near the downflowing fluid bed particles, thereby substantially eliminating the amount of new carbonaceous material which is rapidly entrained within the upflowing gases that are eventually discharged from the combustion chamber.
The present invention also provides many additional advantages which shall become apparent as described below.